(1) Field of the Invention
The present invention relates to a locking screw having peculiar threads for preventing a screw from loosening after a screw having a male screw at a leg thereof is screwed into a female screw hole so that a component part is fixed to a workpiece whose lower hole has a female screw previously formed thereat.
(2) Description of Related Art
In general, a digital camera, a mobile phone, a television game machine, and the like mostly adopt a small screw of M4 or less, in particular, out of so-called small screws having a nominal designation M10 of thread or less during assembling work. The use of these small screws is likely to be avoided because the use of a tapping screw that is to be screwed while forming a female screw possibly induces a deficient product such as a product for short-circuiting an electronic circuit caused by chips produced in forming the female screw since a product to be assembled incorporates therein a precise electronic component part. In view of this, a screw to be used in such a product is mostly a normal small screw under present circumstances.
However, as shown in FIG. 5, the above-described general small screw is designed to exhibit a locking function by the effect of a frictional resistance produced by the contact between a flank surface 121 of a female screw 120 and a flank surfaces 111 of a male screw 110 with each other, as shown in FIG. 6B, by an axial force generated between a seat surface of a head 102 and the flank surface 111 of the male screw 110 after tightening a screw during work for fixing a component part B to a workpiece W. The locking function can be exhibited when the seat surface of the head of a small screw 101 is screwed without suffering from any screw floating. However, initial looseness occurs in the case where the flank surfaces 111 and 121 are separated from each other due to vibrations exerted on the product, the shortage of tightening torque, or the like even if the separation is slight, as shown in FIG. 6A. After that, a locking force cannot be produced, and therefore, the screw 101 is slipped out of a screw hole 104. Under present circumstances, an adhesive, not shown, is applied to the male screw 110 at a leg 103 of the screw 101 in such a manner as to inhibit such looseness, and thus, the screw is used.
A screw having a male screw formed into a shape shown in FIG. 4 has been used in order to produce a locking function on a small screw with respect to a female screw. Such a screw is disclosed in Japanese Patent Application Laid-open No. Sho 63-176810. As shown in FIG. 4, an expanding portion 213 on a chasing side and an expanding portion 214 on an advance side that expand outward and have a predetermined thickness are formed at a crest of a thread at both flank surfaces of a thread, that is, a flank surface 211 on a chasing side and a flank surface 212 on an advance side at the crest of the thread formed around a shaft of a male screw 210. In this manner, locking portions are formed into a substantially rectangular cross-sectional shape at the crest. With this configuration, when this male screw is screwed into a female screw hole formed at a nut or a workpiece, the expanding portions 213 and 214 at the locking portion bite a root at the thread of the female screw with interference. Thus, the frictional resistance occurring at the bitten portion can prevent the male screw from loosening.
However, in the small screw whose leg has the adhesive adhering thereto so as to prevent any looseness in a generally wide use, the adhesive must be always applied to the leg of the screw before the screwing work, thereby requiring the applying process. In order to eliminate this process, the adhesive need be previously applied to the leg of the screw. For the purpose of this, special machining is needed to hold fluidity for a long period of time by preventing the adhesive from being dried. This induces an increase in cost. Moreover, in the case where the screw is slantwise screwed or the screw is floating so as to induce deficient screwing, when the deficiency is to be corrected, the adhesive has been already cured, and therefore, the screw cannot be loosened to be detached. If the screw is forced to be loosened, the head of the screw is crushed or the product is broken, thereby causing deficiency in the product. In the meantime, when the male screw having the expanding portions shown in FIG. 4 for the purpose of the prevention of the looseness is screwed into the female screw hole previously formed at the workpiece, the screw is screwed while the corners of the expanding portions are brought into contact with the female screw, and therefore, a scraping flaw occurs at the flank surface of the female screw, thereby raising a fear of a crush of the female screw. Moreover, in the case where the screw is screwed by an automatic driver such as an electric driver, the mismatch between the tip of the male screw and the female screw at the beginning of the screwing may crush the female screw. In addition, when the workpiece is made of a relatively soft material such as an aluminum alloy or a magnesium alloy, string-like chips are produced. The chips may fall on a wiring board for electronic equipment, and therefore, may adversely influence on an electronic component part, thus causing a deficient product. On the other hand, when the male screw is firmly tightened in the case where the workpiece is not made of a soft material, a tightening force from the flank surface on a female screw press-contact side concentrates on the expanding portion at the flank surface of the chasing side in press contact with the flank surface of the tightened workpiece. Consequently, the expanding portion at the flank surface on the chasing side of the male screw is plastically deformed into a flat surface along the flank surface on the female screw press-contact side. When this screw is repeatedly used, it can exhibit a great locking force for the first time. However, the expanding portion remains plastically deformed during the repeated use. Thus, a problem to be solved such as the degradation of the locking force still remains.